This invention is directed to an improvement in emulsion copolymerization of two comonomers at least one of which is a gas, and which have different solubilities in water.
Higuchi et al, showed in an article in J. Pharm. Sci. (1962) 51,459 that a slightly water-soluble organic compound well dispersed in an aqueous emulsion will gradually diffuse into the aqueous phase, and the emulsion will thus become destabilized unless a small amount of another material insoluble in water but completely miscible with the organic compound is added to stabilize the emulsion.
U.S. Pat. No. 4,113,687 (to Ugelstad) describes a process for preparing stable emulsions of vinyl monomers having a limited water solubility, wherein in the first step a material having a very low solubility in water (such as hexadecane) is emulsified by an efficient homogenization, and then water and the desired vinyl monomer are added. The monomer diffuses rapidly into the preformed droplets and is polymerized to large particles.
In the aqueous emulsion copolymerization of ethylene and vinyl acetate in a stirred reactor, where the monomers, catalyst, surfactant, and water are fed continuously, and the partially converted polymer latex is removed continuously, the copolymer composition changes as a function of time because of the variation of ethylene concentration in the polymer particles. Stryker et al. reported in J. Polymer Sci., Part C; 27, 35-48 (1969) that when a small amount of t-butyl alcohol is added to the aqueous phase in ethylene homopolymerization, the concentration of ethylene in the homopolymer particles remains constant. This is explained by an increase of the rate of transport of gaseous ethylene to the locus of polymer formation to a level where the concentration of emulsified ethylene at the locus of polymerization is constant throughout even an extended residence time.
When two monomers are copolymerized in an aqueous emulsion, at least one of which is a gas, and each has a different solubility in water, the more water-soluble monomer will diffuse more rapidly to the polymer particles and will thus be present at a higher concentration at the polymerization locus. The resulting copolymer will usually contain a higher proportion of the more water-soluble monomer than could be predicted from the respective monomer feed rates and polymerization rates. It thus would be desirable to carry out emulsion copolymerization of two or more comonomers having different water solubilities under conditions which would permit good control of monomer concentrations at the locus of polymerization and thus would result in copolymers having a broader range of comonomer ratios.